1. Field of the Invention
The present invention relates to improvements in a structure and method for mounting a drum unit and a developing unit in an electrophotographic recording apparatus so as to secure a uniform developing gap between a photosensitive drum and a sleeve.
2. Description of the Related Art
As well known, in an electrophotographic recording apparatus, such developing agent as toner is applied from a developing unit to electrostatic latent images formed on a photosensitive drum in a series of electrophotographic recording processes to thereby realize picture images.
A developing gap between the photosensitive drum and a sleeve provided in the developing unit in the developing process exercises a great effect on the quality of a picture image to be recorded through the subsequent transfer and fixing processes.
Thus, for the purpose of securing a good quality of recording picture image, it is necessary to keep the developing gap uniform at all times.
Meanwhile, there are at present a fairly large number of electrophotographic recording apparatuses wherein a drum unit is provided separately from a developing unit. Even for this type of apparatuses, however, these recording units must be mounted in the apparatus so as to secure such a uniform developing gap as mentioned above between a photosensitive drum and a sleeve, as a matter of course.
Brief explanation will next be directed to an ordinary method of keeping the developing gap uniform in this electrophotographic recording apparatus by referring to FIGS. 8 and 9.
In FIG. 8, a photosensitive drum 1 having a rotational axis 1a, which is provided in a drum unit, rotates in such a direction as shown by an arrow A.
A sleeve 2, which is provided in a developing unit, has the same cylindrical shape as the photosensitive drum 1.
The sleeve 2 having a rotational axis 2a parallel to the axis 1a of the drum 1, rotates in such a direction as shown by an arrow B following the rotation of the drum 1.
Provided at left and right ends of the sleeve 2 are roller-shaped collars 3 which have respectively a diameter larger than that of the sleeve 2 and have the common central axis 2a.
With such a structure, a predetermined difference in outer diameter between the sleeve 2 and the collars 3 corresponds to a developing gap g to be explained later.
In this case, the collars 3 loosely fittedly receive the sleeve 2 about the central axis 2a and respectively rotate independently of the sleeve 2.
When mounted in the recording apparatus, the photosensitive drum 1 and the sleeve 2 are made close to each other until the collars 3 provided at the left and right ends of the sleeve 2 are brought into tight contact with the peripheral surface of the drum 1 and the axes 1a and 2a of the drum 1 and sleeve 2 are kept parallel to each other.
Shown in FIG. 9 is the above positional relationship between the drum 1 and sleeve 2 when viewed from such a direction as shown by an arrow C in FIG. 8.
Under such a condition, the photosensitive drum 1 and the sleeve 2 define therebetween the developing gap g which corresponds to the aforementioned diameter difference between the sleeve 2 and collars 3 as shown in the drawing.
In this type of prior art electrophotographic recording apparatus, the above method has been employed for the purpose of obtaining the uniform developing gap g and the actual mounting of the drum unit and developing unit into the apparatus has been realized with such a structure as shown in FIG. 10.
More specifically, in FIG. 10, a rotary shaft 4a parallel to the central axis 2a of the sleeve 2 is provided at a position different from the central axis 2a in a developing unit 4 so that the developing unit 4 can be suspended from the rotary shaft 4a.
In this case, when the developing unit 4 is rotated by its weight on the rotary shaft 4a, this causes the collars 3 to be tightly contacted with the photosensitive drum 1, so that the uniform developing gap g can be obtained between the drum 1 and sleeve 2 by the above method.
In the case of such a mounting structure of these recording units, however, since the central axes 1a and 2a of the photosensitive drum 1 and sleeve 2 are provided respectively as fixed, these members 1 and 2 are frequently biased in their axes when subjected to an external force or the like.
In case the rotary axis 4a of the developing unit 4 is once biased with respect to the central axis 1a for example, these axes will never regain their parallel relation thereafter.
In such a situation, one or both of the two collars 3 provided at the both ends of the sleeve 2 are disposed as floated from the photosensitive drum 1. This disadvantageously results in that it becomes impossible to secure the uniform developing gap between the photosensitive drum 1 and sleeve 2 along their longitudinal direction, which leads to remarkable deterioration of the quality of recording picture image.